Sintered compacts made from cubic boron nitride (cBN) are used in cutting tools and are known for their good wear resistance. To form such a compact, the cBN particles are sintered at high pressure and high temperature (HPHT sintering) to produce a polycrystalline cubic boron nitride (PCBN) structure. The cBN particles may be HPHT sintered in the presence of a substrate material, which provides a metal catalyst that infiltrates into the cBN layer from the substrate and assists with the formation of PCBN and the intercrystalline bonding between the cBN grains.
Alternatively, the cBN particles may be HPHT sintered without a substrate present, in which case the resulting PCBN compact may be described as “solid” or “self-sintering” or “self-sintered” or “free-standing.” A catalyst/binder material may be mixed with the cBN particles prior to sintering in order to promote the formation of the PCBN structure during HPHT sintering, or the catalyst/binder material may be placed adjacent the cBN particles. Sintering without a substrate can be advantageous, as the substrate does not occupy valuable working space within the high pressure press, and the space can be fully occupied by the cBN mixture. As an example, a high pressure press may have a working volume of about 50 cm3. Also, the catalyst/binder material may be uniformly mixed throughout the cBN mixture, rather than infiltrating into the cBN layer from a substrate, and as a result the self-sintered PCBN compacts may have more uniform compositions and material properties.
However, the known catalyst/binder materials used for self-sintering create PCBN compacts that are ceramic (dielectric) in nature and not conductive. As a result the self-sintered PCBN compact cannot be cut by electric discharge machining (EDM). After sintering, it is often necessary to cut the sintered PCBN compact into a desired shape for a particular cutting tool. Cutting by EDM is advantageous in many applications, as EDM cutting can reduce tool processing costs and allow for more precise geometries to be produced. The laser cutting process may produce a less uniform surface finish and less flat (i.e., planar) or perpendicular cut surfaces, resulting in additional finishing costs. The laser cutting process can also cause thermal damage. However EDM cutting requires that the material being cut is conductive or semi-conductive.
The binder materials used to form self-sintered PCBN compacts have typically not been conductive, and therefore the resulting PCBN compact cannot be cut by EDM. For example, one binder precursor material that has been used to form self-sintering PCBN is Aluminum. After HPHT sintering the resulting self-sintering PCBN compact has an Aluminum ceramic binder phase between the PCBN grains. This Aluminum ceramic binder phase is non-conductive. PCBN compacts with other types of binder phases have been attempted in the past, but such compacts have been typically limited to low cBN content, are not EDM-cuttable, and/or do not have sufficient hardness and strength properties for the intended applications.
Accordingly there is still a need for a high cBN content self-sintering PCBN compact with a conductive or semi-conductive ceramic binder phase, that is EDM-cuttable, with desired material properties for an intended application.